One way of cleaning and restoring solid-state materials contaminated by radiocesium, especially soil, is to stabilize cesium found in a stratum or seabed that contains carbonate by using calcium chloride solution to keep cesium from being re-fluidized by groundwater (Patent Document 1). Other presented methods include a method for extracting cesium using acid under increased pressure and heat (Non-patent Document 1), a method for vaporizing cesium in the gas phase by heating it to 800° C. under aeration with vapor (Non-patent Document 2), a method for extracting cesium using carbonated water (Non-patent Document 3), and a method for extracting cesium using oxalic acid (Non-patent Document 4).
The method of Patent Document 1 is an insolubilization technology for cesium, not a removal technology. Of the methods of Non-patent Documents 1 to 4, which are removal technologies for cesium, the methods of Non-patent Documents 1 and 2 require a high temperature and pressure or steaming/baking under high temperature, necessitating the use of a large energy for processing lots of soil. Additionally, Non-patent Documents 3 and 4 do not disclose the processing conditions in detail, and the present inventors retested the process using the actual contaminated soil (fine-grained faction) under common conditions to find that the decrease in the radiation dose remained around 10 to 33% (refer to the Comparative Examples).
It is considered that volume reduction of soil contaminated by radiocesium is possible using a method of washing/classification according to the method for processing solid-state material contaminated by heavy metal, and directly re-burying the coarse-grained fraction (gravel, sand), which is considered to have relatively low specific surface area and relatively low contamination concentration; but, the process leaves the fine-grained fraction (silt, clay) as contamination-concentrated soil. For a mass removal of soil, the fine-grained fraction soil must also undergo cesium desorption and decontamination. In addition, the finding of crushed stone contaminated by a high concentration of radiocesium (i.e. maximum of 214,000 becquerel/kg) at the rock quarry in the Fukushima Prefecture in February 2012 spread the understanding that the coarse-grained fraction obtained after the soil has been washed must also undergo cesium desorption and decontamination.
Coarse-grained fraction (gravel, sand) obtained after the washing/classification process include silicate minerals such as amphibole, andesite, feldspar, calcite; amorphous minerals; and silt and clay minerals formed by partial weathering of the surface of the preceding minerals. Of these, the mineral considered as the major agent in cesium adsorption is the clay mineral developing on or attached to the surface of the sand gravel. The fine-grained fraction (silt, clay) consists of minerals, such as quartz, cristobalite, feldspar, calcite, and layered clay minerals such as amorphous mineral and micas, smectites, vermiculites. Of these, the minerals recognized as the major agent in cesium adsorption are the layered clay minerals, such as amorphous mineral and micas, smectites, and vermiculites (Non-patent Document 5).
It is reported that 70% of cesium in soil is adsorbed in a strongly fixed state to the clay mineral, 20% thereof is adsorbed to an organic material, and the remaining 10% is present in an ion-exchange state (Non-patent Document 6).
The layered clay mineral involved in the cesium adsorption is referred to as the 2:1 layered silicate; it contains thin sheet-like layers stacked on top of each other; and it has negative charge. Cations, such as hydrogen ion, potassium ion and sodium ion, are inserted between layers to cancel the negative charge, and the cesium ion is adsorbed in an ion-exchange state to the end surface portion between layers or the outer surface of the layers (layer surface area) (FIG. 1). The problem is that the tetrahedron arrangement of silica (SiO4) in the layered silicate sheet is structured with continuous openings of a suitable size (adsorption sites) to entrap cesium ion or ammonium ion. When such ions enter the end surface portion between layers, they are gradually become dehydrated (a phenomenon in which water molecules around the ions are detached) and enter the above adsorption sites (Non-patent Document 7). The dehydration flattens the sheet to a unit sheet thickness (i.e., the total thickness of one sheet and the space between layers. It varies according to the space between layers) of approximately 1 nm from approximately 1.5 nm when hydrated ions are contained, and inhibits hydrated ions from physically entering the sheet. That is how cesium is adsorbed in a fixed state to clay mineral, preventing the above 70% of cesium from being extracted even when a high concentration of hydrogen ion (acid) and sodium ion (alkali) is supplied, since the ions are hydrated ions. That is the main reason why the removal ratio of cesium from soil after a usual washing by water, acid or alkali remains at approximately 10 to 30%, and no effective solution to the problem is found yet.
In addition, the soil that adsorbed cesium, especially the above fine-grained fraction having an extremely small particle size of 75 μm or lower, tends to disperse or effuse by natural causes such as wind and rain or human causes such as the decontamination work. Such soil flows into gutters, sewages and rivers to be carried into a filter plant or a sewage plant, and ends in the sludge discharged from the plants where it becomes the main radioactive contaminant of sludge and its burned ash. In addition, the soil accumulates at the bottom of rivers, lakes and harbors (especially at a high concentration around the mouth of the river) where it becomes the main radioactive contaminant for the sediment (hedoro).
The main contaminated matter of sludge in water supply and sewage systems and its burned ash, sediments in rivers, lakes, drainage systems, water ways, canals, harbors and the like, and hedoro is the same as that of soil, or is derived from soil. Hence, when the above sludge, sediments and hedoro are subjected to the usual washing by water, acid or alkali, the removal ratio of cesium remains at approximately 10 to 30%.